1. Field of Invention
This invention pertains to well casings of the type that are utilized in subterranean drilling for and pumping of natural resources such as oil and gas.
2. Related Art and Other Considerations
The world remains greatly dependent upon subterranean natural resources such as oil and gas for energy production. To this end, improved methods and apparatus are continually sought for extracting fluids such as oil and gas from underground formations.
A typical oil or gas well includes an elongated pipe string made up of a plurality of casing assemblies, each of which has both a cylindrical inner pipe and a concentric outer member. Both the outer member and the inner pipe are perforated. The perforations in the outer member allow inflow from a formation into an annular region between the outer member and the inner pipe. The inner pipe has a screen covering its perforations so that particulate matter entrained in the fluid will be removed from the fluid before the fluid flows from the annular region into the inner pipe and is directed axially through the inner pipe to the surface for fluid recovery.
The perforations on the outer member are normally of a sufficiently large size, such as 0.25", so as to not retain sand from the formation. Perforations of such a size would usually protect an internal screen member located beneath the perforations from being damaged. However, in some formations, sharp projections, such as in pockets of shale, can cause damage to an underlying screen if they should penetrate the perforations on the outer member and deform a portion of the screen surface, causing an enlarged opening therein. This is particularly true when the well is deviated at an angle to the vertical, or horizontal, causing the outer member to bear on the formation wall with a considerable force as it is run into the well bore. Moreover, in situations of high fluid flow, the fluid entering through the perforations of the outer member can impinge on certain areas of the screen with sufficient velocity and force so as to damage the screen. For a screen formed by rods and wires, for example, too great a force of the incoming particulate containing fluid applied to a small area of the screen can cause enlargement of the slot openings formed by the wires. Such enlargement would not only reduce the effectiveness of the screen but would cause a reduced pressure drop in the area, and thus cause even more particulate containing flow to impinge on the wires so as to produce further widening and thereby render the screen useless for its intended purpose.
In order to reduce the energy of fluid forces applied to screens in well casing assemblies, it has been proposed to have the outer member provided with slots or openings which are shaped so as to cause the fluid flowing radially through the openings to move a short distance axially before it again flows radially inwardly through a screen member. In this regard, see U.S. Pat. No. 5,624,560 to Voll et al. However, the structure shown in this patent does not address the problem of screens being damaged when protrusions of subterranean formations enter the perforations and deform the underlying screen surface. While it has been known in the prior art to form inlet and outlet perforations at offset locations with respect to concentric pipes, as disclosed in U.S. Pat. No. 1,604,386 to Byerly, perforation placement has not been taught or suggested for protection of screens located inside casing assemblies.
What is needed, therefore, and an object of the present invention, is apparatus for protecting well screens from damage otherwise occasioned by protrusion and/or fluid force. Another object is to provide improved flow distribution in high flow sections of the formation so as to reduce erosion of the well screens.